Light emitting diode chip

ABSTRACT

A light emitting diode chip includes a semiconductor layer sequence having an active layer that generates electromagnetic radiation, wherein the light emitting diode chip has a radiation exit area at a front side, the light emitting diode chip has a mirror layer at least in regions at a rear side situated opposite the radiation exit area, said mirror layer containing silver, a protective layer is arranged on the mirror layer, and the protective layer comprises a transparent conductive oxide.

TECHNICAL FIELD

This disclosure relates to a light emitting diode chip.

WO 2008/131735 A1 discloses a light emitting diode chip wherein a firstand second electrical connection layer are arranged at a rear side ofthe light emitting diode chip situated opposite the radiation exit areaand are electrically insulated from one another by a separating layer,wherein a partial region of the second electrical connection layerextends from the rear side through a perforation of the active layer inthe direction toward the front side of the light emitting diode chip.Making contact with a semiconductor chip in this way has the advantagethat the radiation exit area can be free of contact areas and,consequently, the emitted radiation is not shaded.

The light emitting diode chip is a so-called “thin-film” light emittingdiode chip, wherein the original growth substrate of the semiconductorlayer sequence is detached and, instead, the semiconductor layersequence connects to a carrier by a solder layer at an opposite siderelative to the original growth substrate. In the case of a thin-filmlight emitting diode chip of this type, it is advantageous if that sideof the semiconductor layer sequence facing the carrier is provided witha mirror layer to deflect radiation emitted in the direction of thecarrier in the direction of the radiation exit area and thereby toincrease radiation efficiency.

For the visible spectral range, silver, in particular, is suitable as amaterial for the mirror layer. Silver is distinguished by a highreflection in the visible spectral range and produces a good electricalcontact to the semiconductor material. On the other hand, however,silver is susceptible to corrosion and migration of the silver intoadjacent layers can occur.

To protect a mirror layer composed of silver against corrosion,generally a protective layer is applied to the silver layer. By way ofexample, a platinum layer is suitable as a protective layer. However, ithas been found that the platinum can penetrate into the silver layer atthe process temperatures customary for applying the layers and can evenpass as far as the opposite interface between the mirror layer and thesemiconductor layer. The reflection of the interface between the mirrorlayer and the semiconductor layer sequence can be impaired as a result.This has the consequence that the light coupling-out and thus theefficiency of the light emitting diode chip are reduced. Furthermore,the electrical properties can also change as a result of the diffusionof platinum to the interface between the semiconductor layer sequenceand the mirror layer.

It could therefore be helpful to provide a light emitting diode chipcomprising a rear-side mirror layer protected against corrosion by aprotective layer, wherein the reflection and electrical properties ofthe interface between the silver layer and the semiconductor layersequence are not impaired.

SUMMARY

We provide a light emitting diode chip including a semiconductor layersequence having an active layer that generates electromagneticradiation, wherein the light emitting diode chip has a radiation exitarea at a front side, the light emitting diode chip has a mirror layerat least in regions at a rear side situated opposite the radiation exitarea, said mirror layer containing silver, a protective layer isarranged on the mirror layer, and the protective layer comprises atransparent conductive oxide.

We also provide a light emitting diode chip including a semiconductorlayer sequence having an active layer that generates electromagneticradiation, wherein the light emitting diode chip has a radiation exitarea at a front side, the light emitting diode chip has a mirror layerat least in regions at a rear side situated opposite the radiation exitarea, said mirror layer containing silver, a protective layer isarranged on the mirror layer, the protective layer comprises atransparent conductive oxide, and the mirror layer adjoins thesemiconductor layer sequence at an interface situated opposite theprotective layer.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a schematic illustration of a cross section through a lightemitting diode chip in accordance with one example.

DETAILED DESCRIPTION

The light emitting diode chip may contain a semiconductor layer sequencehaving an active layer that generates electromagnetic radiation. Thelight emitting diode chip has a radiation exit area at a front side,through which radiation exit area the electromagnetic radiation emittedby the active layer emerges from the semiconductor layer sequence. Thefront side of the light emitting diode chip is understood here andhereinafter to be that side of the light emitting diode chip at whichthe radiation exit area is arranged.

At a rear side situated opposite the radiation exit area, the lightemitting diode chip has a mirror layer at least in regions, the mirrorlayer containing silver or preferably consisting of silver.

A protective layer that reduces corrosion of the mirror layer isarranged on the mirror layer. The protective layer advantageouslycontains a transparent conductive oxide (TCO) or consists thereof. Wefound that a transparent conductive oxide is particularly well suited toprotect the mirror layer against environmental influences and/ordiffusion of constituents of adjacent layers, wherein the material ofthe transparent conductive oxide does not diffuse into the mirror layerand, in particular, does not diffuse through the mirror layer as far asan interface—situated opposite the protective layer—between the mirrorlayer and semiconductor layer sequence. Therefore, the mirror layeradvantageously contains no material of the protective layer.

The optical and electrical properties of the interface between themirror layer and the semiconductor layer sequence are therefore notimpaired by diffusion of material of the protective layer as far as theinterface. In particular, the high reflection at the interface betweenthe mirror layer composed of silver and the semiconductor layer sequenceis not reduced.

The protective layer composed of the transparent conductive oxideadvantageously also prevents diffusion of the silver from the mirrorlayer into layers which succeed the protective layer at the rear side ofthe light emitting diode chip such as an electrical contact layer, forexample.

The material of the transparent conductive oxide can advantageously beselected and/or controlled with regard to a high electricalconductivity. This is based on the fact that the protective layer isadvantageously arranged on a side of the mirror layer facing away fromthe semiconductor layer sequence and, consequently, is not situated inthe beam path of the light emitting diode chip. For this reason, it isnot necessary for the material of the transparent conductive oxide tohave a high transparency. In particular, conductivity of the transparentconductive oxide can be improved by addition of a dopant.

An interface of the mirror layer situated opposite the protective layerpreferably adjoins the semiconductor layer sequence. Therefore, betweenthe semiconductor layer sequence and the mirror layer there is arranged,in particular, no intermediate layer such as an adhesion promoter layer,for example, which might result in a reduction of the reflection at theinterface between the mirror layer and the semiconductor layer sequence.The mirror layer can adjoin, in particular, a p-type semiconductorregion of the semiconductor layer sequence.

Preferably, the transparent conductive oxide comprises a zinc oxide(ZnO) or particularly preferably a doped zinc oxide, for example, ZnO:Alor ZnO:Ga. Further preferably, the transparent conductive oxide is anindium tin oxide (ITO), indium zinc oxide (IZO) or indium gallium zincoxide (IGZO). These transparent conductive oxides are distinguished, inparticular, by a good electrical conductivity.

The protective layer preferably has a thickness of 5 nm to 500 nm,particularly preferably 10 nm to 100 nm.

Preferably, the side flanks of the mirror layer and/or of the protectivelayer are covered by an electrically insulating layer. The electricallyinsulating layer electrically insulates the side flanks of the mirrorlayer and of the protective layer from layers adjacent in a lateraldirection. Furthermore, the electrically insulating layer protects, inparticular, the side flanks of the mirror layer against corrosion.

The electrically insulating layer can comprise, for example a siliconoxide, a silicon nitride or a silicon oxynitride. The electricallyinsulating layer is particularly preferably a SiO₂ layer.

Further preferably, a first electrical connection layer is arranged atan opposite side of the protective layer relative to the mirror layer.The first electrical connection layer is preferably formed from at leastone material having good electrical conductivity to impress the currentas uniformly as possible into the semiconductor layer sequence.Consequently, the first electrical connection layer also functions as acurrent spreading layer. The first electrical connection layerelectrically conductively connects to one of the electrical contacts ofthe light emitting diode chip.

The first electrical connection layer is preferably formed from aplurality of partial layers, wherein the partial layers advantageouslycomprise, proceeding from the mirror layer, a platinum layer, a goldlayer and a titanium layer. In this case, the partial layer composed ofplatinum advantageously functions as a diffusion barrier preventing thediffusion of constituents of succeeding layers into the protective layeror even into the mirror layer, and vice versa. The partial layercomposed of gold functions as a current spreading layer on account ofthe high electrical conductivity, and the succeeding titanium layerfunctions as an adhesion promoter layer for further succeeding layers.

The light emitting diode chip preferably connects to a carrier at anopposite side relative to the semiconductor layer sequence as viewedfrom the mirror layer. The carrier is, in particular, a substratedifferent than a growth substrate of the semiconductor layer sequenceand connected to the semiconductor layer sequence by a solder layer, forexample.

A growth substrate used for the epitaxial growth of the semiconductorlayer sequence is preferably detached from the light emitting diodechip. Therefore, the light emitting diode chip preferably has no growthsubstrate. By virtue of the fact that the growth substrate is detachedfrom the light emitting diode chip and the radiation emitted in thedirection of the carrier is reflected toward the radiation coupling-outarea by the mirror layer, a light emitting diode chip having a highefficiency is obtained.

Preferably, the light emitting diode chip has a first and a secondelectrical connection layer, wherein the first and second electricalconnection layers face the rear side of the semiconductor layer sequenceand are electrically insulated from one another by an electricallyinsulating layer, wherein a partial region of the second electricalconnection layer extends from the rear side of the semiconductor layersequence through at least one perforation of the active layer in thedirection toward the front side. In this configuration, therefore, bothelectrical connection layers are advantageously arranged at the rearside of the light emitting diode chip. This has the advantage, inparticular, that a radiation exit area of the light emitting diode chipcan be free of connection contacts. The electrically insulating layerelectrically insulating the first and second connection layers from oneanother can cover the side flanks of the mirror layer and/or of theprotective layer and in this way also protects the layers againstexternal influences. The electrically insulating layer advantageouslycomprises an oxide or nitride such as, for example, a silicon oxide, asilicon nitride or a silicon oxynitride.

Our chips are explained in greater detail below on the basis of anexample in connection with FIG. 1.

The constituent parts illustrated and also the size relationships of theconstituent parts among one another should not be regarded as true toscale.

The light emitting diode chip 1 illustrated in FIG. 1 has asemiconductor layer sequence 2 having an active layer 3 that emitselectromagnetic radiation 13.

The active layer 3 of the light emitting diode chip 1 can be, forexample, a pn-junction, a double heterostructure, a single quantum wellstructure or multiple quantum well structure. In this case, thedesignation quantum well structure encompasses any structure in whichcharge carriers experience a quantization of their energy states as aresult of confinement. In particular, the designation quantum wellstructure does not include any indication about the dimensionality ofthe quantization. It therefore encompasses, inter alia, quantum wells,quantum wires and quantum dots and any combination of these structures.

The semiconductor layer sequence 2, can, in particular, be based on anitride compound semiconductor. In this context, “based on a nitridecompound semiconductor” means that the semiconductor layer sequence 2 orat least one layer thereof comprises a III nitride compoundsemiconductor material, preferably In_(x)Al_(y)Ga_(1−x−y)N, where 0≦x≦1,0≦y≦1 and x+y≦1. In this case, this material need not necessarily have amathematically exact composition according to the above formula. Rather,it can comprise one or more dopants and additional constituents whichsubstantially do not change the characteristic physical properties ofthe In_(x)Al_(y)Ga_(1−x−y)N material. For the sake of simplicity,however, the above formula only includes the essential constituents ofthe crystal lattice (In, Al, Ga, N), even if these can be replaced inpart by small amounts of further substances.

The light emitting diode chip 1 emits electromagnetic radiation 13through a radiation exit area 4 arranged at the front side of the lightemitting diode chip 1. The radiation exit area 4 can be provided with aroughening or a coupling-out structure (not illustrated) to improveradiation coupling-out.

The light emitting diode chip 1 is a thin-film light emitting diodechip. In the thin-film light emitting diode chip, the original growthsubstrate of the semiconductor layer sequence 2 has been detached fromthe light emitting diode chip 1 and, instead, the light emitting diodechip 1 has been connected to a carrier 19 by a solder layer 18 at anopposite side relative to the original growth substrate. The originalgrowth substrate may have been detached, in particular, from thatsurface of the semiconductor layer sequence 2 which now functions as theradiation exit area 4. In the light emitting diode chip 1, therefore, ann-type semiconductor region 2 a usually first grown onto the growthsubstrate faces the radiation exit area 4. A p-type semiconductor region2 of the semiconductor layer sequence 2 faces the carrier 19. Thecarrier 19 can comprise germanium or silicon, for example.

The light emitting diode chip 1 has a mirror layer 5 in regions at arear side situated opposite the radiation exit area 4 to improve theefficiency of the light emitting diode chip 1. The mirror layer 5 isarranged between the p-type semiconductor region 2 b and a firstelectrical connection layer 10 of the light emitting diode chip.Radiation emitted by the active layer 3 in the direction of the carrier19 is advantageously reflected toward the radiation exit area 4 by themirror layer 5.

The mirror layer 5 advantageously contains silver or consists thereof. Amirror layer 5 composed of silver advantageously has a high reflectionin the visible spectral range. Furthermore, silver is distinguished by ahigh electrical conductivity. The mirror layer 5 can adjoin the p-typesemiconductor region 2 b, in particular, and in this way form one of theelectrical connections of the semiconductor layer sequence 2 of thelight emitting diode chip 1.

In the case of a mirror layer 5 composed of silver, the problem canoccur that the layer is comparatively susceptible to corrosion, whichmight result in a reduction of the radiation efficiency particularlyafter a long operating duration of the light emitting diode chip 1. Toprotect the mirror layer 5 against corrosion and prevent diffusion ofmaterial of the mirror layer 5 into succeeding layers 7, 8, 9, and viceversa, a protective layer 6 is arranged on the interface of the mirrorlayer 5 facing away from the semiconductor layer sequence 2.

The protective layer 6 advantageously comprises a transparent conductiveoxide. Preferably, the protective layer 6 contains ZnO, ZnO:Ga, ZnO:Al,ITO, IZO or IGZO or consists thereof

In comparison with a metallic protective layer such as platinum ortitanium, for example, the protective layer 6 composed of thetransparent conductive oxide has the advantage that the material of theprotective layer 6 does not diffuse into the mirror layer 5.

Consequently, the material of the protective layer 6, in particular,does not reach the interface—situated opposite the protective layer6—between the semiconductor layer sequence 2 and the mirror layer 5. Areduction—caused by diffusion of material of the protective layer 6 intothe mirror layer 5—of the reflection and/or a change in the electricalproperties, in particular the forward voltage, therefore advantageouslydo(es) not occur. Furthermore, the protective layer 6 advantageouslyalso prevents diffusion of the silver from the mirror layer 5 intosucceeding layers 7, 8, 9 and diffusion of material of the succeedinglayers 7, 8, 9, in particular a metal such as gold, for example, intothe mirror layer 5.

The protective layer 6 advantageously has a thickness of 5 nm to 500 nm,particularly preferably 10 nm to 100 nm.

At an interface situated opposite the mirror layer 5, the protectivelayer 6 adjoins the first electrical connection layer 10. The firstelectrical connection layer 10 can have a plurality of partial layers 7,8, 9. By way of example, the first electrical connection layer 10 hasthree partial layers. Proceeding from the protective layer 6, thepartial layers are preferably a platinum layer 7, a gold layer 8 and atitanium layer 9. The platinum layer 7 has, in particular, the functionof a diffusion barrier which prevents the diffusion of constituents ofthe succeeding layers 8, 9 into the protective layer 6, or even into themirror layer 5, and vice versa. The gold layer 8 functions as a currentspreading layer on account of the high electrical conductivity, and thesucceeding titanium layer 9 functions as an adhesion promoter layer forfurther succeeding layers, in particular an electrically insulatinglayer 14.

The light emitting diode chip 1 additionally has a second electricalconnection layer 15, by which contact is made with the light emittingdiode chip 1 from a rear side situated opposite the radiation exit area4. Therefore, both the first electrical connection layer 10 and thesecond electrical connection layer 15 are arranged at a rear side of thelight emitting diode chip 1 facing the carrier 19. This has theadvantage that the radiation exit area 4 is free of electricalconnection layers such that the electromagnetic radiation 13 emitted bythe light emitting diode chip 1 is not shaded by one of the electricalconnection layers 10, 15.

The first electrical connection layer 10 preferably makes contact withthe p-type region 2 b of the semiconductor layer sequence 2. The secondelectrical connection layer 15 preferably makes contact with the n-typeregion 2 a of the semiconductor layer sequence 2. For this purpose, thesecond electrical connection layer 15 is led from the rear side of thelight emitting diode chip 1 through one or more perforations 21 a, 21 bextending through the p-type region 2 b of the semiconductor layersequence and the active layer 3, right into the n-type region 2 a of thesemiconductor layer sequence 2. The second electrical connection layer15 is electrically insulated from the active layer 3 and the p-typeregion 2 b of the semiconductor layer sequence 2 in the region of theperforations 21 a, 21 b by an electrically insulating layer 14 toprevent a short circuit.

Furthermore, the electrically insulating layer 14 also insulates thesecond electrical connection layer 15 from the first electricalconnection layer 10. The electrically insulating layer 14 is, forexample, an oxide or nitride layer, preferably a silicon oxide layer, asilicon nitride layer or a silicon oxynitride layer.

The electrically insulating layer 14 advantageously covers, at least inpartial regions, the side flanks 16 of the mirror layer 5 and of theprotective layer 6. In this way, the mirror layer 5 is advantageouslyalso protected against corrosion at the side flanks 16.

The second electrical connection layer 15 preferably contains silver andfunctions, in particular in the region of the perforations 21 a, 21 b,as a second mirror layer for the electromagnetic radiation 13 emitted bythe light emitting diode chip 1.

A diffusion barrier layer 17 is preferably arranged between the secondelectrical connection layer 15 and the solder layer 18. The diffusionbarrier layer 17 prevents a constituent of the solder layer 18 fromdiffusing into the second electrical connection layer 15, and viceversa. The diffusion barrier layer 17 contains titanium tungstennitride, for example.

The light emitting diode chip 1 is soldered onto the carrier 19 by thesolder layer 18. The solder layer 18 can contain AuSn, in particular.

The carrier 19 of the light emitting diode chip 1 can be, for example, agermanium or silicon carrier. A contact metallization 20 can be appliedat the rear side of the carrier facing away from the light emittingdiode chip 1, via which contact metallization the second electricalconnection layer 15 is electrically connected toward the outside.

The first electrical connection layer 10 can be electrically connectedtoward the outside, for example, via a bonding pad 11 and a bonding wire12.

Our chips are not restricted by the description on the basis of theexamples. Rather, this disclosure encompasses any novel feature and alsoany combination of features, which in particular includes anycombination of features in the appended claims, even if the feature orcombination itself is not explicitly specified in the claims orexamples.

The invention claimed is:
 1. A light emitting diode chip comprising asemiconductor layer sequence having an active layer that generateselectromagnetic radiation, wherein the light emitting diode chip has aradiation exit area at a front side, the light emitting diode chip has amirror layer at least in regions at a rear side situated opposite theradiation exit area relative to the active layer, said mirror layercontaining silver, a protective layer is arranged on the mirror layer,the protective layer comprises a transparent conductive oxide, a firstelectrical connection layer is arranged at an opposite side of theprotective layer relative to the mirror layer, the first electricalconnection layer is formed from a plurality of partial layers, and theplurality of partial layers comprise, proceeding from the minor layer, aplatinum layer, a gold layer and a titanium layer.
 2. The light emittingdiode chip according to claim 1, wherein the mirror layer adjoins thesemiconductor layer sequence at an interface of the mirror layersituated opposite to the protective layer relative to the mirror layer.3. The light emitting diode chip according to claim 1, wherein theprotective layer comprises ZnO, ZnO:Ga, ZnO:Al, ITO, IZO or IGZO.
 4. Thelight emitting diode chip according to claim 1, wherein the protectivelayer has a thickness of 5 nm to 500 nm.
 5. The light emitting diodechip according to claim 4, wherein the protective layer has a thicknessof 10 nm to 100 nm.
 6. The light emitting diode chip according to claim1, wherein side flanks of the mirror layer and/or of the protectivelayer are covered by an electrically insulating layer at least inregions.
 7. The light emitting diode chip according to claim 6, whereinthe electrically insulating layer is an oxide or nitride layer.
 8. Thelight emitting diode chip according to claim 1, wherein the lightemitting diode chip connects to a carrier, and the carrier is oppositeof the semiconductor layer sequence relative to the mirror.
 9. The lightemitting diode chip according to claim 1, wherein the light emittingdiode chip comprises no growth substrate.
 10. The light emitting diodechip according to claim 1, wherein the light emitting diode chipcomprises a first electrical connection layer and a second electricalconnection layer, the first electrical connection layer and the secondelectrical connection layer face the rear side of the semiconductorlayer sequence and are electrically insulated from one another by anelectrically insulating layer, and a partial region of the secondelectrical connection layer extends from the rear side of thesemiconductor layer sequence through at least one perforation of theactive layer in a direction toward the front side.
 11. The lightemitting diode chip according to claim 1, wherein a region of thesemiconductor layer sequence adjoins the mirror layer, and is a p-typesemiconductor region.